Color schemes should avoid losing information content

[hallpell]

Some color schemes for the code can leave out information, especially the 'matching parenthesis' bit. Monokai, 3024-night, base16-dark, and midnight just have an underline for the matching paren which is much easier to miss.

Also some issues with bolding that other people brought.

It might make sense to either remove some of the less clear color schemes, or see if we can change the styling a bit to emphasize common student issues.

[hallpell]

Based on #615 , this can be dealt with by the css files provided by codemirror. I'm considering having our own copies of them that modify the matching-bracket property to be more obvious/obnoxious for some of the schemes that are currently more subtle. Diverging from the codemirror code base gives me pause, but I think we'll already be going away from it enough that this won't lose anything important.